The most common ways that herbicides injure off-target plants are spray drift and herbicide volatility. Spray drift is the physical movement of airborne spray particles during the application, that never settle on the target vegetation. Volatility is the tendency of a chemical to vaporize into the atmosphere from a treated surface even after it has been deposited on the target.
Spray drift is affected by a number of factors: nozzle type, application volume, wind speed and adjuvants, to name a few. To minimize herbicide drift, it’s recommended that droplets be larger than 400 microns, as smaller droplets can be easily carried off-site. Obviously high wind speeds increase the likelihood of drift, however when wind speed is less than 2 miles per hour, wind direction is highly variable and can change direction suddenly. That’s why Dow AgroSciences always recommends a wind speed range for optimum spray deposition. And remember, off-target damage potential increases when wind blows toward susceptible crops. Some states have state regulations for proper wind speeds during applications.
Herbicide volatility varies by product and can occur with applications on any surface, but increases on impervious surfaces like rocks and pavement. The main climatic situation where volatility occurs is during periods of high temperatures and low humidity, and calm conditions can contribute to volatility. While drift can occur with any herbicide, Dow AgroSciences offers many products that are classified as essentially non-volatile, the lowest classification utilized by the EPA to describe the volatility potential of a herbicide.